linux-kernel - Re: [benchmark] 1% performance overhead of paravirt

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Message-ID: <20090609164519.GE9211@wotan.suse.de>
Date:	Tue, 9 Jun 2009 18:45:19 +0200
From:	Nick Piggin <npiggin@...e.de>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Rusty Russell <rusty@...tcorp.com.au>, Ingo Molnar <mingo@...e.hu>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Avi Kivity <avi@...hat.com>,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: [benchmark] 1% performance overhead of paravirt_ops on native kernels

On Tue, Jun 09, 2009 at 09:26:47AM -0700, Linus Torvalds wrote:
> 
> 
> On Tue, 9 Jun 2009, Nick Piggin wrote:
> > 
> > The idea seems nice but isn't the problem that kmap gives back a
> > basically 1st class kernel virtual memory? (ie. it can then be used
> > by any other CPU at any point without it having to use kmap?).
> 
> No, everybody has to use kmap()/kunmap().

So it is strictly a bug to expose a pointer returned by kmap to
another CPU? That would make it easier, although it would need
to remove the global bit I think so when one task migrates CPUs
then the entry will be flushed and reloaded properly.


> The "problem" is that you could in theory run out of kmap frames, since if 
> everybody does a kmap() in an interruptible context and you have lots and 
> lots of threads doing different pages, you'd run out. But that has nothing 
> to do with kmap_atomic(), which is basically limited to just the number of 
> CPU's and a (very small) level of nesting.

This could be avoided with an anti-deadlock pool. If a task
attempts a nested kmap and already holds a kmap, then give it
exclusive access to this pool until it releases its last
nested kmap.

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